This invention relates to a fluid displacement apparatus, and more particularly, to an improvement in a rotation preventing/thrust bearing device for an orbiting member of a fluid displacement apparatus.
There are several types of fluid displacement apparatus which utilize an orbiting member, such as a piston or a scroll, driven by a Scotch yoke type shaft coupled to an end surface of the orbiting member. One such apparatus, disclosed in U.S. Pat. No. 1,906,142 issued to John Ekelof, is a rotary machine which has an annular and eccentrically movable piston adapted to act within an annular cylinder. The annular cylinder has a radial transverse wall, one end of which is fixedly mounted. Another wall forms a cover disc which is connected to the annular piston; the annular piston is driven by a crank shaft. Other prior art apparatus of this type, which consists of scroll type fluid displacement apparatus, is shown in U.S. Pat. Nos. 801,182 and 3,500,119. Though the present invention applies to either an annular orbiting piston or an orbiting scroll, the description will be limited to a scroll type compressor.
The above mentioned U.S. Pat. No. 801,182 (Creux) discloses a device including two scrolls each having a circular end plate and a spiroidal or involute spiral element. These scrolls are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scrolls shifts the line contacts along the spiral curved surfaces and, as a result, the volume of the fluid pockets changes. Since the volume of the fluid pockets increases or decreases dependent on the direction of the orbital motion, the scroll type fluid displacement apparatus is applicable to compress, expand or pump fluids.
Generally, in conventional scroll type fluid displacement apparatus, one scroll is fixed to a housing and the other scroll, which is the orbiting scroll, is eccentrically supported on a crank pin of a rotating shaft to cause orbital motion. The scroll type apparatus also includes a rotation preventing device which prevents rotation of the orbiting scroll to thereby maintain the scroll in a predetermined angular relationship during operation of the apparatus.
Because the orbiting scroll in conventional scroll type apparatus is supported on a crank pin in a cantilever manner, an axial slant of this orbiting scroll occurs. Axial slant also occurs because the movement of the orbiting scroll is not rotary motion around the center of the orbiting scroll, but is orbiting motion caused by the eccentric movement of a crank pin driven by the rotation of the drive shaft. Several problems result from the occurrence of this axial slant including improper sealing of line contacts, vibration of the apparatus during operation and noise caused by physical striking of the spiral elements. One simple and direct solution to these problems is the use of a thrust bearing device for carrying the axial loads. Thus, a scroll type fluid displacement apparatus is usually provided with a thrust bearing devicewithin the housing.
One recent attempt to improve the rotation preventing and thrust bearing devices in scroll type fluid displacement apparatus is described in U.S. Pat. Nos. 4,160,629 (Hidden et al) and 4,259,043 (Hidden et al). The rotation preventing and thrust bearing functions in these U.S. patents are integral with one another. A rotation preventing/thrust bearing device according to these patents comprises one set of indentations formed on the end surface of the circular plate of the orbiting scroll and a second set of indentations formed on the end surface of fixed plate attached to the housing. A plurality of balls or spheres are placed between the indentations of both surfaces. All the indentations have the same cross-sectional configuration and the center of all the indentations formed on both end surfaces are located on circles having the same radius. As a result, the machining and fabrication of these indentations with accurate dimensions is very difficult and intricate.
Referring to FIGS. 1 and 2, one prior art solution to overcome the above disadvantages is shown. FIG. 1 is a vertical sectional view of a part of a compressor and FIG. 2 is an exploded perspective view of a rotation preventing/thrust bearing device. Rotation preventing/thrust bearing device 37' surrounds a boss 273' of orbiting scroll 27' and includes an orbital portion, a fixed portion and bearings, such a plurality of balls. The fixed portion includes (1) an annular fixed race 371' having one end surface fitted against an axial end surface of annular projection 112' of front end plate 11', and (2) a fixed ring 372' fitted against the other axial end surface of fixed race 371' to extend outwardly therefrom and cover the other end surface of fixed race 371'. Fixed race 371' and ring 372' are attached to the axial end surface of annular projection 112' by pins 373'. The orbital portion also includes (1) an annular orbital race 374', which has one end surface fitted against an axial end surface of circular plate 271' and (2) an orbital ring 375' fitted against the other axial end surface of orbital race 374' to extend outwardly therefrom and cover the other axial end surface of orbital race 374'. A small clearance is maintained between the end surface of fixed ring 372' and the end surface of orbital ring 375'. Orbital race 374' and ring 375' are attached to the end surface of circular plate 271' by pins 376'.
Fixed ring 372' and orbital ring 375' each have a plurality of holes or pockets 372a' and 375a' in the axial direction, the number of holes or pockets in each ring 372' and 375' being equal. The holes or pockets 372a' on fixed ring 372' correspond to or are a mirror image of the holes or pockets 375a' on orbital ring 375', i.e., each pair of pockets facing each other have the same size and pitch, and the radial distance of the pockets from the center of their respective rings 372' and 375' is the same, i.e., the centers of the pockets are located at the same distance from the center of the rings 372' and 375'. Thus, if the centers of the rings 372' and 375' were aligned, which they are not in actual operation of the rotation preventing/thrust bearing device 37', the holes or pockets 372a' and 375a' would be identical or in alignment. Bearing elements, such as balls or spheres 377' are placed between facing or generally aligned pairs of pockets 372a' and 375a' of fixed and orbital rings 372' and 375'.
In this arrangement, the radii of pockets 372a' and 375a' are equal to half the orbital radius Ro. Also, during operation of the compressor, balls 377' are held within the pockets by the edges of opposing pairs pockets, i.e., balls 377' move along the edges of opposing pairs pockets. Thus, when orbiting scroll orbits at radius Ro, the traveling radius of balls 377' against the race is half the traveling radius of the orbiting scroll during orbital motion. Since the width of the race must be greater than the traveling radius of balls, the size of the race is determined by the traveling radius of the balls. The race on the orbiting scroll contributes to the total weight of the orbital part, which includes the orbiting scroll, the orbital portion of the rotation preventing/thrust bearing device, and part of the driving mechanism. Therefore, it is desirable to reduce the size of the race on the orbiting scroll in order to reduce the total weight of the orbital part of the prior art compressors.